Problems have existed in the synthesis of polyconjugated dienes broadly and specifically in the synthesis of polybutadiene, including: the reduction the polymer cement viscosity in the polymer product; suppression of gel formation in the reaction mass; and, the reduction of a broad weight distribution in the polymer product.
The use of various catalyst systems containing rare earth compounds for the polymerization of conjugated diene monomers has been disclosed. Examples of such disclosures are (1) Mazzei A., Makromol. Chem. Suppl. 4 61 (1981); (2) Witte J., Angew. Makromol. Chem. 94 119 (1981); (3) Shen Tse-Chuan et al, J. Pol. Sci. Polym. Chem. Ed. 18 3345 (1980); (4) Marwede G. and Sylvester G., Trans. 22nd Annual Proceedings of the International Institute of Synthetic Rubber Producers, Madrid Paper III-3 (1981). Such catalyst systems have two or three components, for example a lanthanoid alkyl, alkoxide or salt (e.g. neodymium tricarboxylate) with an organoaluminium compound and optionally a Lewis Acid. When used in the polymerization of conjugated dienes, they yield a product which has a high content of cis isomer (e.g. 98% for polybutadiene and 94% for polyisoprene)
Diethylzinc (hereinafter referred to as "Et.sub.2 Zn") is well known as a chain transfer agent in the Zeigler-Natta polymerizations as described in J. Boor, "Zeigler-Natta Catalysts and Polymerizations", 1979, p375. Diethylzinc has also been used in BuLi and barium catalyzed polymerizations of butadiene as described in H. Hseih, J Polym. Sci., Polym. Chem. Ed., Vol. 14, p379-386, 1976; and, U.S. Pat. Nos. 4,129,705 and 4,092,268, to de Zaraux. EP Patent Application No. 0234 512 to Takeshi, teaches that Et.sub.2 Zn can be used in combination with rare earth catalyzed polymerizations of butadiene, as a third, nonessential catalyst component of the system. However, Takeshi makes no disclosure that Et.sub.2 Zn may be used reduces the viscosity of the polymerization.
Thus, there exists a long felt need to vary the molecular weight of polybutadienes during synthesis; more particularly, to reduce the polymer cement viscosity of polybutadienes during their synthesis. The instant invention purports to solve all of the foregoing problems without noticeably affecting the rate of the polymerization or the microstructure of the resultant polymer.